1. Field of the Invention
The present invention relates generally to the field of heat exchange in industrial furnace boilers and in particular to an industrial package boiler having improved furnace gas temperature regulation due to a balanced heat flux between the burner heat release rate and the boiler heat absorption rate to minimize the creation of thermal NO.sub.X during the combustion process.
2. Description of the Related Art
Industrial power generation furnaces and boilers are well known for being a source of pollutants such as nitrogen and sulfur oxide compounds (NO.sub.X and SO.sub.x), which are created during the combustion process. In recent years, many solutions for reducing the amounts of these pollutants into the atmosphere have been set forth. One method of reducing the amount of pollutants released is to simply reduce the amount of NO.sub.X and SO.sub.X created by the furnace combustion process.
Control of nitrogen oxide emissions (NO.sub.X) in particular is important in the operation of industrial power generation furnaces and boilers, and other apparatus which use natural gas burners. Attempts to control NO.sub.X emissions have focused primarily upon the creation of "thermal NO.sub.X ", which is a portion of the total NO.sub.X emissions generated by combustion. Thermal NO.sub.X is formed by the oxidation of molecular nitrogen (N.sub.2) at high temperatures.
Generally, when water tube boilers are fired with gas fuel, they are fired with diffusion type burners.
Circular burners are one type of diffusion burner which are frequently referred to where the combustion is initiated as the fuel and the combustion air pass through the circular throat. The result is a rapid mixing of fuel and air and almost immediate combustion with quick heat release creating high furnace gas temperatures, usually in the range of 2800.degree. F. to 3000.degree. F., and large amounts of NO.sub.X as the heat of combustion is liberated more rapidly than the water cooled furnace can absorb.
Several techniques for reducing the amount of NO.sub.X generated in diffusion burners have been devised, including methods which delay the combustion until the water cooled surface has a chance to absorb some of the heat and/or reducing combustion gas temperatures by changing the mass of the gas flow. However, the furnace gas temperatures still escalate to levels above the threshold of thermal NO.sub.X creation. Further, the burner can become unstable if the intermediate combustion process is delayed too long, or if the mass flow is increased too much.
Radiant burners are a surface burner type which can operate at much lower levels of NO.sub.X generation because much of the heat of combustion is radiated to the water cooled furnace walls and does not result in elevated combustion flue gas temperatures. One kind of radiant burner is a pre-mix burner in which gaseous fuel and combustion air are mixed prior to entry to the burner. Radiant burners have a relatively low heat release capacity, in the range of 100 kBtu/ft.sup.2 -hr of burner surface. The combustion process is very stable, even with rapid load changes. Despite this and other known benefits, capital costs are usually very high for use in large capacity boiler designs. One solution for overcoming high NO.sub.X generation is provided in U.S. Pat. No. 5,439,372, in which multiple radiant burners are fired in multiple zones to increase the total heat release rate while maintaining a lower NO.sub.X generation rate.
Multiple firing rate zone (MFRZ) burners have achieved a higher heat release rate than the radiant burners--in the range of 1,250 kBtu/ft.sup.2 -hr of burner surface. The NO.sub.X generation rate is also higher than that of a radiant burner, to which this type of burner is related. The combustion rate is higher than radiant burners and the combustion gas temperatures are elevated above the thermal NO.sub.X threshold before the water cooled furnace walls can absorb the extra heat. Multiple firing rate zone burners are also pre-mix burners and have a very stable combustion. U.S. Pat. No. 5,439,372 discloses one multiple firing rate zone burner for producing low NO.sub.X emissions.